The present invention relates to a liquid crystal display apparatus and, more particularly, to an active matrix type liquid crystal display apparatus driven by thin film transistors.
As image display apparatuses capable of being made thin in thickness, light in weight and high in precision, market of thin film transistor drive liquid crystal display apparatuses (hereinafter, referred to as TFT-LCD) is growing wider compared to the conventional cathode lay tube displays. The TFT-LCD comprises gate wires and data wires formed on a glass substrate; thin film transistors, each of the transistors being formed at a position near an intersecting point of the gate wire and the data wire; pixel electrodes connected to the thin transistors; a gate insulator film; an insulating protective film; an opposite substrate; and a liquid crystal layer interposed between the glass substrate and the opposite substrate. With the recent trend of larger screen and higher precision of the TFT-LCD, requirement to improving properties of the wiring such as low resistivity, low stress, high workability is being increased. Metallic films are generally used for the gate wires and the data wires, but the conventional simple substance such as Al or Cr can not satisfy the requirement. Japanese Patent Application Laid-Open No. 5-184747 discloses that a film made of an Al--Nd alloy containing Nd composition of 5.1 to 48.5 wt % (1 to 15 at %) is employed for TFT-LCD wiring. In addition, Japanese Patent Application Laid-Open No.7-4555, Japanese Patent Application Laid-Open No.8-18060, Japanese Patent Application Laid-Open No.8-306693 and Japanese Patent Application Laid-Open No.1-289140 disclose Al--Nd alloys or lanthanide alloys. Further, Japanese Patent Application Laid-Open No.10-199827 discloses a laminated wiring of ITO and Al.
The TFT-LCD has an advantage that it is thin in thickness and light in weight compared to the cathode lay tube type display apparatus, but has a problem in that its production yield is likely decreased and its cost is high because number of its manufacturing processes is large and its structure is complex. In detail, the TFT-LCD has a structure composed of plural kinds of thin films such as scanning signal wires (gate wiring films), a gate insulator film, a semiconductor film, data wires (source, drain wiring film), pixel electrode films, opposite electrode films, an insulator protective film and so on having a thickness of several tens to several hundreds nano-meters on a TFT substrate. The thin films are formed through a photo-lithographic process, and it is difficult to attain a high accuracy in the patterning work over the whole area and to suppress occurrence of short-circuiting between the electrodes and break in the wiring. In addition, the basic property of the wiring in connection with the larger screen and the high precision is required to be a low resistance so as to eliminate unevenness in display. This requirement can be satisfied by using a low resistivity wiring material or by using thick wires since the resistance is in direct proportion to a resistivity of the wiring material and in inverse proportion to a thickness of the film. It is ideal that the wiring material is of a low resistivity, but actually, the resistivity of the general wiring material is not satisfactorily low. Therefore, the low resistance wire capable of eliminating unevenness of display in a TFT-LCD can be attained by using thick wires. In a case of using the thick wires, it is important that edge portions of the wire are formed in a forward tapering shape. Here, the forward tapering shape means a shape that a cross-sectional shape of the wire formed on a substrate is wider in the interface between the wire and the substrate and narrower in the obverse surface side opposite to the substrate side of the wire. The TFT-LCD includes a structure that an insulator film is interposed between a lower electrode and an upper electrode. Particularly, in a case where the forward tapering shape can not formed in the lower electrode, an insulating property of the insulator film is deteriorated in a portion of the insulator film covering a step of the lower electrode, and short-circuiting between the lower electrode and the upper electrode may occur, which is one of main causes to decrease the manufacturing yield of the TFT-LCD.
Aluminum is a low resistivity and low stress material. However, thermal stress is apt to cause hillocks because of the low melting point, and accordingly a problem of short-circuiting is apt to cause even on the surface of the wire. As a material which makes use of the low resistivity and low stress of aluminum and is difficult to cause the hillocks, Japanese Patent Application Laid-Open No.5-184747 discloses that the Al--Nd alloy containing Nd composition of 5.1 to 48.5 wt % (1 to 15 at %) is employed for TFT-LCD wiring. However, in a case where the wires are made thicker with the trend of a larger screen and a higher precision, the manufacturing yield of the TFT-LCD will be decreased due to the problem in that short-circuiting between the upper and lower electrodes occurs at the position of the insulator film covering the step in the lower electrode, and due to a problem in that a defect occurs in the upper electrode pattern by penetration of an etching solution for treating the upper electrode into cracks in the insulator film deteriorated its insulating property of the insulator film during patterning treatment of the upper electrode. That is, the alloy satisfies the hillock resistance, but improvement of the manufacturing yield is not taken into consideration.